Non-invasive micro-rheology of a single bacterium-scale filament of extracellular polymeric substances using oscillating flow*

While extracellular polymeric substances (EPS) account for up to 90% of biofilm mass, their material properties are difficult to measure due in part to their spatial heterogeneity within biofilms and delicate structure that is easily disrupted by invasive rheological techniques. Here we present a non-invasive micro-rheological technique to measure material properties of a single filament of EPS with sparsely attached bacteria in an oscillating flow. A bacterial suspension flows through a microfluidic channel containing a single pinned submillimeter oil droplet on which bacteria attach and secrete EPS. The EPS extrudes into a long filament (or streamer) with thickness approximately equal to one bacterium. Using high speed microscopy, single bacteria trapped in the filament are tracked to determine the filament deformation. Concurrently, freely suspended bacteria are used as flow tracers to perform PIV-assisted PTV to measure highly resolved velocity fields and determine viscous stresses and pressures experienced by the filament as the flow oscillates. Stress-strain relationships are developed with the tell-tale hysteresis of viscoelastic materials. Deformation at both oscillation and mean flow time scales is observed.